03 400ex trouble

[ToothDoc]

O.K. 400 ex with K-n-N airfilter White Brothers E-series exhaust stock jetting.Ran great for a while now the plug seems to foul,It is very black and dry/Does anyone know whats up?/Its hard to start and some backfiring.

[ReconRider25]

did you change the plug when you put the exhaust etc. on? if you didnt you wont get a good reading. sure its still black from before when the jetting is good, but if it was a new plug itd be a white color and the backfiring is from running lean. you need to rejet